1. Field of the Invention
The present invention relates generally to a driver circuit, and more particularly, to a low output voltage driver circuit for driving a light-emitting device and a related error detection circuit.
2. Description of the Prior Art
A traditional constant current driver, as shown in FIG. 1, is generally employed for generating a driving current for driving a load device. The current driver, especially a light-emitting diode (LED) driver, usually requires high output impedance. High output impedance can be obtained either by decreasing channel length modulation (increasing the length of the driver MOSFET) or by cascading the driver (which is illustrated in FIG. 2). The former one trade-offs between the current accuracy and the circuit area, and thus this is not an effective way of obtaining high output impedance since high accuracy trades off with the slew rate. The later one employs negative feedback loop to increase the output impedance by a factor of approximately 2+gm*ro. In the case of negative feedback loop, the output impedance is increased through the gain loop, so that devices (MOSFETs) having shorter channel length may be used. However, this type of circuit configuration will not operate satisfactorily when the output voltage of the current driver is quite low. In addition, a current mirror circuit configuration having an active feedback is disclosed by U.S. Pat. No. 6,194,967 to Johnson et al. As disclosed, an operational amplifier is utilized in the current mirror circuit configuration such that the corresponding voltages at terminals or ports of the disclosed current mirror circuit configuration are substantially identical. However, the output part of the disclosed current mirror circuit configuration consists of two cascoded transistors, and these two cascoded transistors increase the output voltage level, leading to the increasing of power consumption. Additionally, a large driver area due to cascoded transistor results large parasitic capacitance loading thus reduces slew rate. Moreover, when the cascoded transistors are operation in triode region, it is not just ameliorates the effect due to variation of threshold voltage as described in the prior art. Further, it disadvantageously aggravates variation of the output current caused by the output voltage. As a consequence, the cascoded transistors cannot provide the output current with high accuracy. Even though the current mirror configuration can control the gate voltages precisely, it fails to provide the output current with high accuracy to the load device.
Therefore, there are still some shortcomings in the traditional design of the current driver that require to be improved.